Memory devices are typically provided as internal, semiconductor, integrated circuit devices in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory has developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage (Vt) of the memory cells, through programming (which is often referred to as writing) of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data state (e.g., data value) of each memory cell. Common uses for flash memory and other non-volatile memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones, and removable memory modules, and the uses for non-volatile memory continue to expand.
A NAND flash memory is a common type of flash memory device, so called for the logical form in which the basic memory cell configuration is arranged. Typically, the array of memory cells for NAND flash memory is arranged such that the control gate of each memory cell of a row of the array is connected together to form an access line, such as a word line. Columns of the array include strings (often termed NAND strings) of memory cells connected together in series between a pair of select gates, e.g., a source select transistor and a drain select transistor. Each source select transistor may be connected to a source, while each drain select transistor may be connected to a data line, such as column bit line. Variations using more than one select gate between a string of memory cells and the source, and/or between the string of memory cells and the data line, are known.
Memory cells are typically erased before they are programmed to a desired data state. For example, memory cells of a particular block of memory cells may first be erased and then selectively programmed. For a NAND array, a block of memory cells is typically erased by grounding all of the access lines (e.g., word lines) in the block and applying an erase voltage to the channels of the memory cells (e.g., through data lines and source connections) in order to remove charges that might be stored on charge storage structures (e.g., floating gates or charge traps) of the block of memory cells. Typical erase voltages might be on the order of 25V before completion of an erase operation. For programming of these memory cells, one or more programming pulses are typically applied to a selected access line connected to a control gate of each memory cell associated with that selected access line. Typical programming pulses might start at or near 15V and may increase in magnitude during each subsequent programming pulse application. Programming pulses on the order of 25V might be experienced before completion of a programming operation. While the programming pulse is applied to the selected access line, a ground potential might be applied to the channels of the memory cells, resulting in a charge transfer from the channel to the charge storage structures of memory cells targeted (e.g., selected) for programming.